This invention relates to a method and apparatus for tracking the varying resonant frequency of an electrically resonant structure. More particularly the invention is directed towards applications where said structure is mounted remotely from the driving and sensing electronics.
The prior art most closely related to that of the present invention is the technique for determining the excitation frequency of rubidium electrons in a rubidium gas cell atomic clock (as described in model 304BR Rubidium Frequency Standard, Tracor Inc. 1968). The rubidium atoms in the closed gas cell are excited by an alternating electromagnetic field of very high frequency. If the frequency exactly matches the spin frequency of the electrons in the outer shell of the rubidium atoms, the electrons will change energy state. To measure this change, optical radiation is shone through the cell and detected by a photo diode. The light intensity is at a minimum when the atoms are excited at the correct frequency. To detect this minimum, the excitation frequency is swept a small amount either side of resonance and the output of the photo detector is applied as feedback to the exciting frequency to allow the minimum light intensity to be maintained.
Although the basic philosophy of this prior art is similar to the present invention, the technical field of the invention is completely divorced and the method of implementation is completely different. The object of this invention is to track the resonant frequency of an electrically resonant structure whose resonant frequency varies continuously with time. An example of such a structure is a surface acoustic wave (SAW) resonator used as a strain or temperature sensor as described in our European Patent 0518900.
The first aspect of the present invention is an apparatus for tracking the varying resonant frequency of an electrically resonant structure, characterised in that it comprises a variable frequency oscillator providing an excitation signal of a variable frequency encompassing the possible resonant frequency range of said resonant structure, a bidirectional RF transmission line connecting said variable frequency oscillator and said resonant structure, a directional coupler incorporated in the transmission line which generates a directional coupler signal proportional to the reflected signal from said resonant structure, said directional coupler signal being conditioned by a processor to provide a feedback signal to the input of the variable frequency oscillator, such that the mean frequency of said excitation signal is caused to continuously track the varying resonant frequency of said resonant structure.
Preferably the tracking of said varying resonant frequency involves at least two functions, a first search function for initially searching the possible resonant frequency range of said resonant structure and, once resonance of said resonant structure is established, a second following function for following said variable resonant frequency as it varies as a function of time.
Preferably, in a first embodiment, said processor comprises a detector which detects said directional coupler signal, a synchronous rectifier for selectively rectifying the output of the detector relative to phase a frequency source having at least one master frequency output driving said synchronous rectifier, an integrator for conditioning the output from said synchronous rectifier, said integrator being input into a summer which also receives a second input from said frequency source for modulating the feedback signal to said variable frequency oscillator, thereby providing said following function It is preferred that said second input from said frequency source is synchronised with said master frequency output of said frequency source.
Preferably said frequency source also provides a search frequency output which is substantially lower than said master frequency output. The search frequency output being input into said integrator and therefore causing said variable frequency oscillator to sweep through said possible resonant frequency range of said resonant structure in the absence of any feedback signal.
Preferably the output of said variable frequency oscillator is monitored to indicate the resonant frequency of said electrically resonant structure. Alternatively or additionally, an analogue signal from the output of said integrator or summer may be used indicate the resonant frequency of said resonant structure.
Preferably, in a second embodiment, said processor comprises a double balance mixer which receives said directional coupler signal from said directional coupler and said excitation signal from said variable frequency oscillator and provides a phase proportional DC output to an integrator for conditioning the output of said integrator providing said feedback signal to the variable frequency oscillator, thereby providing said following function.
Preferably the electrically resonant structure is at least partially composed of piezoelectric material. Suitable piezoelectric materials include quartz and directionally orientated zinc oxide.
Preferably the electrically resonant structure is electrically excited by means of at least one interdigital array (IDA). Suitable resonant structures which incorporate IDAs are surface acoustic wave (SAW) resonators, shallow bulk acoustic wave (SBAW) resonators or the like. Preferably the variable impedance of the resonant structure results from the variation in the pitch of the IDA or mass loading of the resonant structure.
Preferably the variation in the pitch of the IDA results from strain of the resonant structure.
Preferably the resonant structure is substantially rigidly mounted to a surface subject to strain, and this strain is therefore imparted to said resonant structure.
Strain of said surface may be caused by physical quantities such as applied load, applied bending moment, applied pressure, or thermal expansion caused by temperature
Mass loading of the resonant structure may be caused by absorption of fluids into the surface of the resonant structure in the presence of specific fluids or by physical quantities such as humidity.
Preferably the directional coupler may be a transformer, Maxwell Bridge (wire line) or Lange coupler.
Preferably the RF transmission-line incorporates a non-contacting in-line coupler, which may be an untuned or tuned transformer, laser, optical, capacitive or RF coupler. Alternatively the RF transmission line is a continuous electrical conductor between the frequency source and the resonant structure.
Preferably the electrically resonant structure is mounted on the surface of a rotating member subject to strain, with the in-line coupler allowing the transmission of said excitation signal and said reflected signal to and from said rotating member respectively, in a non-contacting manner.
Preferably the output impedance of the variable frequency oscillator should be substantially conjugately matched to any one of said bidirectional RF transmission line, resonant structure, directional coupler, and in-line coupler.
The second aspect of the resent invention is a device for measuring differential strain incorporating two or more apparatuses for tracking the varying resonant frequenes of respective two or more electrically resonant structures, characterised in that each tracking apparatus comprises a variable frequency oscillator providing an excitation signal of a variable frequency encompassing the possible resonant frequency range of its respective resonant structure, a bidirectional RF transmission line connecting said variable frequency oscillator and said respective resonant structure, said transmission line incorporating a directional coupler which generates a directionial coupler signal proportional to the reflected signal from said respective resonant structure, said directional coupler signal conditioned by a processor to provide a feedback signal to the input of the variable frequency oscillator, such that the mean frequency of said excitation signal is caused to continuously track the varying resonant frequency of said respective resonant structure.
Preferably the outputs of at least two variable frequency oscillators of said two or more apparatuses provide inputs to a mixer, the output of which is used to indicate a differential frequency which relates of said differential strain.
Preferably the resonant frequencies of said two or more resonant structures of respective said apparatuses differ from one another. Alternatively, said two or more resonant structures may have substantially the same resonant frequency.